Hey mate, you can purchase replacement batteries for your device. Here is the link to one of the ebay listing i found for it https://www.google.com.au/search?q=kindle+4+battery&client=tablet-android-samsung&sa=X&biw=1024&bih=768&tbs=vw:l,ss:44&tbm=shop&prmd=sivn&srpd=11159573600457747453&prds=num:1,of:1,epd:11159573600457747453&ved=0ahUKEwi9he7A9-zRAhUJVZQKHbSwCo8QgjYIugQ
Also, here is a link for a how-to guide, on replacing the battery https://youtu.be/9nHMAAAuyRs
Hope this helps you.